With the introduction of multiple antennas in the realization of MIMO (Multiple Input Multiple Output) systems, the transmission capacity of OFDM (orthogonal frequency division multiplexing) systems is largely expanded as the number of communication channels in space is multiplied. This benefit is derived, however, at the expense of a more complex receiver design compared with the SISO (single input single output) solution. A direct consequence of the MIMO structure that contributes to the added complexity is the need for identification of more spatial channels. More sophisticated channel estimation schemes and more pilot symbols are therefore needed.
Several challenging issues are posed when considering the design of pilots for channel estimation. The most pressing of all is associated with the provision of adequate information for identifying all channel responses between every transmit and receive antenna pair. More preambles certainly allow for more accurate channel estimation and simpler design, but are too expensive to engage due to a reduction in throughput. An excessive rise in overhead can undermine the whole idea of enhancing data throughput in MIMO transmission altogether.
FIG. 1 shows a long preamble 100 according to prior art.
The long preamble 100 is used for the single antenna case and comprises three OFDM symbols 102 of FFT (fast Fourier transform) size NFFT=8 and each OFDM symbol comprises a cyclic prefix (guard interval) 101 of the length NCP=3 modulated symbols.
In this conventional design, one cyclic prefix 101 is provided for each OFDM symbol 102 in the long preamble 100 to protect it against inter-symbol interference (ISI).
An object of the invention is to provide a method for generating long preambles such that the transmission overhead necessary for long preamble transmission can be reduced compared to prior art.
The object is achieved by a transmitter, a method for generating a plurality of long preambles and a communication device with the features according to the independent claims.